各位大牛,在 centos7 中安装 letsencrypt 证书时遇到这个错误怎么解决？

错误代码是
Replacing certbot-auto...
Creating virtual environment...
Installing Python packages...
Traceback (most recent call last):
  File "/tmp/tmp.BZ7PbBwAvJ/pipstrap.py", line 177, in <module>
    sys.exit(main())
  File "/tmp/tmp.BZ7PbBwAvJ/pipstrap.py", line 158, in main
    for path, digest in PACKAGES]
  File "/tmp/tmp.BZ7PbBwAvJ/pipstrap.py", line 117, in hashed_download
    response = opener(using_https=parsed_url.scheme == 'https').open(url)
  File "/usr/lib64/python2.7/urllib2.py", line 437, in open
    response = meth(req, response)
  File "/usr/lib64/python2.7/urllib2.py", line 550, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib64/python2.7/urllib2.py", line 469, in error
    result = self._call_chain(*args)
  File "/usr/lib64/python2.7/urllib2.py", line 409, in _call_chain
    result = func(*args)
  File "/usr/lib64/python2.7/urllib2.py", line 656, in http_error_302
    return self.parent.open(new, timeout=req.timeout)
  File "/usr/lib64/python2.7/urllib2.py", line 437, in open
    response = meth(req, response)
  File "/usr/lib64/python2.7/urllib2.py", line 550, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib64/python2.7/urllib2.py", line 469, in error
    result = self._call_chain(*args)
  File "/usr/lib64/python2.7/urllib2.py", line 409, in _call_chain
    result = func(*args)
  File "/usr/lib64/python2.7/urllib2.py", line 656, in http_error_302
    return self.parent.open(new, timeout=req.timeout)
  File "/usr/lib64/python2.7/urllib2.py", line 431, in open
    response = self._open(req, data)
  File "/usr/lib64/python2.7/urllib2.py", line 449, in _open
    '_open', req)
  File "/usr/lib64/python2.7/urllib2.py", line 409, in _call_chain
    result = func(*args)
  File "/usr/lib64/python2.7/urllib2.py", line 1258, in https_open
    context=self._context, check_hostname=self._check_hostname)
  File "/usr/lib64/python2.7/urllib2.py", line 1214, in do_open
    raise URLError(err)
urllib2.URLError: <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:618)>
应该怎么解决？